1. Field of the Invention
The present invention relates to a method for analyzing the structure of a serotonin transporter (SERT) tracer, and more particular, to a method for analyzing fragmented structures of [123I]ADAM and precursor thereof, SnADAM, and an analytical method for determining the purity of SnADAM.
2. Related Art
Serotonergic neuronal function plays an important role in the central nerve system. Serotonin is mainly produced in the region of the raphe nuclei, and then projected to other brain regions, such as olfactory bulb, cerebral cortex, hippocampus, and basal ganglia. Serotonin transporters (SERTs) are macromolecular complexes and located in the semipermeable membrane of serotonergic neuronal terminals for regulating the neuronal function and content of serotonin, removing serotonin from the synaptic cleft and sending back into the neuronal cytoplasm, where it can be repackaged for reuse or metabolized.
Recent studies show that functions of the serotonergic system are related to different psychiatric and neurological disorders, neurodegenerative disorders, drug addiction, and eating disorders. Neurological disorders include depression, obsessive-compulsive disorder, schizophrenia, anxiety, and autism, and so on. Neurodegenerative disorders include Parkinson's disease, and Alzheimer's disease, and so on. Eating disorders include bulimia nervosa and so on.
In addition, SERTs are also the major targets for antidepressants and anti-obesity drugs, for example, selective serotonin reuptake inhibitors (SSRIs). Studies show that the response to the treatment with these drugs can be predicted according to the availability of SERTs. Even the findings of studies on positron emission tomography (PET) and single photon emission computed tomography (SPECT) show the toxicity of the drug “ecstasy (MDMA)” to serotonergic neurons. Therefore, it is very important to directly detect whether the functions of the human serotonergic system are normal.
Many SERT tracers have been disclosed in the prior art, which are applicable to in-vivo imaging of cerebral neuroreceptors by using PET or SPECT. These SERT tracers include, for example, [11C](+)McN5652, [11C]SAB, [11C]nor-β-CIT, [11C]MADAM, [11C]AFM, [11C]DAPA, [18F]ACF, [18C]AFM, [123I]5-iodo-6-nitroquipazine, [123I]IDAM, [123I]ODAM, [123I]β-CIT, and [123I]nor-β-CIT. However, most of the tracers have the disadvantages of undesired specific binding, pharmacokinetics, selectivity, specificity, or signal transduction property. Up to now, I-123-2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine ([123I]ADAM) is one of the most desirable SERT tracers.
[123I]ADAM has a molecular formula of C15H17N2SI, an average molecular weight (calculated based on non-radioactive [127I]ADAM) of 385.28, and a chemical structure as shown in FIG. 1(A). The synthesis, purification and analysis methods of [123I]ADAM were first developed by Oya et al. from Departments of Radiology and Pharmacology, University of Pennsylvania (Nucl. Med. Biol., 2000, Vol. 27, pp. 249-254). Oya et al. performed a radioiodination through an oxidative iododestannylation reaction to synthesize [123I]ADAM under acidic conditions.
In the above oxidative iododestannylation reaction, a tributyltin compound, 2-((2-((Dimethylamino)methyl) phenyl)thio)-5-(tri-n-butyltin)-phenylamine (SnADAM), is used as a precursor. SnADAM has a molecular formula of C27H44N2SSn, an average molecular weight of 547.43, and a chemical structure as shown in FIG. 1(B).
Currently, ADAM (I-123-ADAM and F-18-ADAM) are mainly developed in the laboratories of Departments of Radiology and Pharmacology, University of Pennsylvania, the United States; Karolinska Institute, Sweden; Institute Nuclear Energy Research, National Yang-Ming University, Chang-Gung University, and Chang-Gung Memorial Hospital, Taiwan.
At first, the synthetic product [123I]ADAM was purified by complex techniques, such as extraction, drying, and high-performance liquid chromatography (HPLC). However, since the eluent used in HPLC contains a large amount of acetonitrile, it is not suitable for direct injection into the human body. Moreover, since iodine-123 has a half-life (T1/2) of only 13.2 hours and a gamma energy of 159 keV, and complex treatment processes need to be used, the risks of drug contamination, radiation dose, degradation of chemical ingredients and decrease in activity of [123I]ADAM are increased. Therefore, Institute of Nuclear Energy Research (INER) of Taiwan developed a fast solid phase extraction (SPE), in which the neutralized reactants are directly poured onto an octyl cartridge for purification. Firstly, the cartridge is eluted with water and 50% ethanol to remove the impurities. Then, [123I]ADAM is eluted with absolute alcohol for subsequent dilution before use. University of Amsterdam and INER adopt octyl cartridges as the cartridges for fast SPE, and University of Pennsylvania adopts C4 minicolumns (Vydac). There is no significant difference between the elution processes for the octyl cartridges and the C4 minicolumns. The advantages of the fast SPE lie in that, concentrated products can be obtained quickly and automatically labeled, thus reducing the radiation dose received by the personnel.
The purified [123I]ADAM is introduced into the human body via intravenous injection and measured by using an SPECT, so as to compare the activity ratio of regions of interest (ROIs) to regions of non-interest (RONIs) (background, BG). The activity ratio is referred to as the specific binding (SB) ratio, as shown in Equation (1):
                              SB          =                                                    A                ROI                            -                              A                BG                                                    A              BG                                      ,                            (        1        )            
in which, AROI is the region of interest (ROI) radioactivity and ABG is the background (BG) radioactivity.
Up to now, all of the published studies concerning the quality assurance analysis of [123I]ADAM are about analytical methods for directly analyzing the radiochemical purity (RCP) of the product [123I]ADAM by using HPLC, and none of them provides an analytical method for analyzing the structure and purity of [123I]ADAM and its labeled precursor, SnADAM. However, the radioactive RCP analysis can only determine the activities of chemical species containing the radionuclide I-123, including radioactive compounds containing ionic I-123 and I-123 bond, but cannot determine the concentrations of non-radioactive compounds, such as SnADAM and other degradation products and impurities.
The parent molecule [123I]ADAM may be fragmented into daughter molecules of different structures due to chemical reaction, especially after [123I]ADAM is introduced into the human body. Therefore, it is important to study whether these daughter molecules have side or adverse effects on the human body or not.